1. Field of the Invention
The present invention relates to a novel optical recording medium and the recording of information thereon. More particularly, the present invention relates to an optical information recording medium, preferably in the form of a disk or in a tape format, suitable for use with optical recording and playback apparatus. The present invention also relates to such optical information media wherein the information layer is comprised of a near-infrared absorbing chromophore.
2. Description of the Prior Art
Optical recording methods in which light from a laser is focused upon the surface of a recording medium with sufficient intensity to cause a detectable change in the physical characteristics of the surface material have been proposed. Among these methods is the establishment of an information pattern of deformations. In such methods, the information representative pattern of deformations may be formed in the surface of the recording medium by suitably controlling the intensity of the focused light in accordance with the information to be recorded while relative motion is established between the recording medium and the focused light spot.
The recording medium, of course, is one of the key elements in any optical information storage system. The commercial viability of the recording medium depends upon such technical parameters as the sharpness in recording and playback of the information, i.e., a high signal to noise ratio. Dyes and pigments have accordingly been employed in information layers, often to enhance the sensitivity of the recording layers at the particular wavelength of the laser being used, which results in a sharper recording and hence playback of the information.
For example, Spong, U.S. Pat. No. 4,097,895, describes a recording medium which comprises a light reflecting material, such as aluminum or gold, coated with a dye-containing light absorbing layer, such as fluorescein, which is operative with an argon laser light source. The thickness of the light absorbing layer is chosen so that the structure has minimum reflectivity. An incident light beam then ablates, vaporizes or melts the dye-containing light absorbing layer, leaving a hole and exposing the light reflecting layer. After recording at the wavelength of the recording light, maximum contrast between the minimum reflectance of the light absorbing layer and the reflectance of the light reflecting layer exists.
Other U.S. patents which disclose the use of a light absorbing dye in the recording layer include U.S. Pat. Nos. 4,412,231 and 4,446,223. The former patent discloses using a mixture of dyes having different light absorbing wavelengths so that the resulting recording layer has a light absorptivity of or more at all the wavelengths in the range of from 400-900 nm. The latter patent discloses an optical information recording element comprising a support coated with a layer of an amorphous composition, which composition comprises a binder and an oxoindolizine or oxoindolizinium dye.
In a paper entitled "Single Wavelength Optical Recording in Pure, Solvent Coated Infrared Dye Layers" by Gravesteijn, Steenbergen and van der Veen, experiments on the use of certain dyes for optical recording for digital and video applications at GaAlAs laser wavelengths are reported. The paper was presented at the Proceeding of the SPIE, "Optical Storage Media", volume 420, June 6-10, 1983. The specific dyes discussed in the paper are squarylium dyes and pentamethine dyes. It is further suggested in the pape that the solubility of the squarylium dyes in organic solvents can be greatly increased by the introduction of t-butyl groups into thiopyrylium end groups.
The use of dyes in conjunction with optical recording media comprising a styrene oligomer is disclosed in the article by Kuroiwa et al appearing in the Japanese Journal of Applied Physics. Vol. 22, No. 2, February, 1983, pp. 340-343. Among the dyes and pigments discussed as being useful is a copper phthalocyanine pigment. Phase separation and incompatibility between the dyes and oligomers were noted in the article as being problems in the use of dyes for optical information media.
Other patents which disclose the use of phthalocyanine compounds in optical recording media include U.S. Patent No. 4,241,355. Described therein is a recording medium comprising a light reflecting layer and a light absorbing layer, which layer absorbs light at from about 750 to 850 nanometers. The absorbing layer is comprised of a phthalocyanine pigment, which can be substituted with a chlorine atom and where the central atom is selected from the group consisting of lead, aluminum, vanadyl, or tin. Example of specific pigments disclosed for use in the recording medium described include lead phthalocyanine, chloroaluminumphthalocyanine, vanadyl phthalocyanine, stannic phthalocyanine, or chloroaluminum chlorophthalocyanine. According to the disclosure of this patent, these phthalocyanine compounds all absorb energy at solid state injection laser wavelengths and thus all can be evaporated onto a light reflecting layer to produce smooth, optical quality light absorption layers that form recorded information having high signal-to-noise ratios. The optical recording media disclosed in this patent were prepared by placing a reflective substrate in a vacuum chamber and causing, for example, the phthalocyanine, such as lead phthalocyanine, to be vacuum evaporated thereon. Note also U.S. Patent No. 4,340,959.
There is described in U.S. Patent No. 4,298,971 an optical recording medium wherein the recording layer consists entirely, or uubstantially entirely, of a compound of a metal oxide, or metal halide phthalocyanine, vacuum vapor deposited onto a substrate with or without a thin film of chalcogenide of tellurium. Examples of suitable phthalocyanines disclosed include vanadyl phthalocyanine, tin phthalocyanine, nickel phthalocyanine, aluminum phthalocyanine, zinc phthalocyanine, platinum phthalocyanine, and the like.
U.S. Patent No. 4,458,004 discloses an optical information recording medium comprising a base and a recording layer formed on the base. The recording layer comprises a fluorine phthalocyanine capable of absorbing light with a wavelength of 700 nm or more. The fluorine phthalocyanine is substituted by at least one fluorine atom on its aromatic rings, and the central group of the compound is selected from the group consisting of hydrogen, a metal, a metal oxide and a metal halide.
Disclosed in U.S. Pat. No. 4,492,750 is an ablative infrared sensitive optical recording composition containing as a component thereof a dispersion of a resinous binder and a soluble naphthalocyanine substituted on its aromatic rings with hydrogen or alkyl groups containing from 4 to about 8 carbon atoms. The central group of the naphthalocyanine compound is two hydrogen atoms, a divalent, trivalent or tetravalent metal complex.
U.S. Pat. No. 4,529,688 discloses an infrared sensitive optical recording composition containing as a component thereof a dispersion of a resinous binder and an absorbing infra-red sensitive tertiary butyl substituted vanadyl phthalocyanine obtained by the vapor treatment of a non-infrared sensitive tertiary butyl substituted vanadyl phthalocyanine of a specified structural formula.
While the foregoing patents disclose many different dyes useful in optical recording, there still remains a need for chromophores which are strong, effective absorbers in the near-infrared regions, i.e., at wavelengths longer than the effective absorption range of phthalocyanines and naphthalocyanines. In particular, compounds showing strong absorption in the range of from 820-830 nm are greatly desired.
Moreover, while dyes or pigments have been employed in the information storage layers of optical recording media due to their excellent absorption properties, problems are still encountered with regard to the application of the dyes or pigments as a stable layer. The addition of dyes to film-forming polymers due to limited solubility of the dye in the polymer and the tendency of the dye/polymer mixture to phase separate over time are severe problems which need to be overcome. The search for an improved information storage medium comprising a dye or pigment overcoming the aforementioned problems is continuously ongoing. What is desired is a recording layer material which of course exhibits a high extinction coefficient, but which also exhibits excellent film-forming properties to enhance its coating applicability, and good solubility in solvents for ease of manipulation. A recording medium which further eliminates the problem of phase separation over time would also be most desirable. Thus, providing a chromophore which absorbs strongly at a desired wavelength is solving only half the problem. Such a compound would not be commercially viable if it suffers from all of the foregoing problems. Excellent stability with respect to thermal actinic and oxidative degradation is also a desirable and necessary feature.
Accordingly, it is an object of the present invention to provide an optical recording medium containing an anthracyanine or phenanthracyanine compound in the recording layer, thereby rendering the recording layer extremely absorptive at wavelengths ranging from 780 to 850 nm.
It is another object of the present invention to provide an optical recording medium employing an anthracyanine or phenanthracyanine chromophore composition which also exhibits good thermomechanical and film forming properties.
It is yet another object of the present invention to provide a novel optical recording medium which allows for ready application of the chromophore to form a stable information layer, while still exhibiting excellent absorption properties in the near-infrared region.
Still another object of the present invention is to provide a novel optical recording medium which contains a chromophore in the information layer, for which the problem of phase separation over time frequently encountered in dye/polymer mixtures is eliminated.
Yet another object of the present invention is to provide a one-component material for use in an information layer of an optical recording medium which exhibits strong film-forming and thermomechanical properties, and excellent absorption properties in the near-infrared region.
These and other objects, as well as the scope, nature and utilization of the invention, will be apparent to those skilled in the art from the following description and the appended claims.